Rewrite DefaultPrefStore for performance

base/prefs/value_map_pref_store.h

Index: base/prefs/value_map_pref_store.h
diff --git a/base/prefs/value_map_pref_store.h b/base/prefs/value_map_pref_store.h
index b8a34ae96ae3623a77d38b1c22a1d5175f530fd8..dc7993978c1074c0d33cff3aa2eee8201059cb42 100644
--- a/base/prefs/value_map_pref_store.h
+++ b/base/prefs/value_map_pref_store.h
@@ -18,9 +18,6 @@
 // storing the preference values.
 class BASE_PREFS_EXPORT ValueMapPrefStore : public PrefStore {
  public:
-  typedef std::map<std::string, base::Value*>::iterator iterator;
-  typedef std::map<std::string, base::Value*>::const_iterator const_iterator;
-
   ValueMapPrefStore();
 
   // PrefStore overrides:
@@ -30,11 +27,6 @@ class BASE_PREFS_EXPORT ValueMapPrefStore : public PrefStore {
   virtual void RemoveObserver(PrefStore::Observer* observer) OVERRIDE;
   virtual size_t NumberOfObservers() const OVERRIDE;
 
-  iterator begin();
-  iterator end();
-  const_iterator begin() const;
-  const_iterator end() const;
-
  protected:
   virtual ~ValueMapPrefStore();